A. Field of the Invention
This invention relates generally to the field of telecommunication and the processes by which voice and digital data are transmitted over telephone networks to and from a customer""s premises. More particularly, the invention is directed a telephone line conditioner device that is installed in the network between the telephone company network and the lines leading to the customer premises. Basically, the telephone line conditioners described herein are devices to improve the transmission quality of the telephone lines. Line conditioners perform several functions, including amplifying and equalizing the transmission line between the network and customer premises. Because line conditioners are a type of signal repeater located at an intermediate point along a two-wire transmission line, they typically separate the full-duplex (i.e., simultaneous and bidirectional) two-wire transmission lines into two two-wire simplex (unidirectional) circuits. The devices thus contain interface circuitry, commonly known as hybrids, to convert the two-wire full-duplex circuit to two simplex circuits (or four-wire circuit), and to convert the four-wire circuit back to a two-wire circuit. Hybrids originally used transformers to connect the two-wire and four wire circuits. Electronic hybrids were also developed to split and recombine the circuits. The line conditioners amplify and equalize the signals in the simplex circuits between the hybrids.
In a typical POTS application, a line conditioner is located at a customer""s residence. The device has two pairs of terminals: one pair is connected to the customer""s equipment (the xe2x80x9ccustomer sidexe2x80x9d or xe2x80x9ceast sidexe2x80x9d); the other pair is connected to the phone company""s equipment (the xe2x80x9cline sidexe2x80x9d or xe2x80x9cwest sidexe2x80x9d). Internally, as indicated above, the line conditioner device has circuitry for the two distinct signal paths: one for signals from the customer side to the line side (or an xe2x80x9ceast-westxe2x80x9d circuit) and one for signals originating from the line side to the customer side (or a xe2x80x9cwest-eastxe2x80x9d circuit). Because each pair of terminals is used both for incoming and outgoing signals, one problem associated with line conditioning circuits is that an outgoing signal being directed towards the two-wire system is also simultaneously detected as an input and is retransmitted in the direction from which it originated, thereby causing an echo. Such echos are highly undesirable because they are distracting to the speaker, and can seriously degrade the performance of communications devices such as computer modems and facsimile machines. Effective line conditioning devices must therefore be capable of minimizing echos.
B. Description of the Prior Art
One prior art technique for preventing or suppressing echos is to allow communications in only one direction at a time. Communications formats of this type are commonly referred to as half-duplex. The conditioners inhibit or mute signals from one side when signal energy is detected from the other side. This has the effect of eliminating the possibility of signals returning to the source, but also prevents an individual from hearing speech, or any other sounds, originating from the other party as long as the individual is speaking. The technique also causes dropouts during the switching of the circuits which tends to disrupt conversations. Additionally, this echo suppression technique must be disabled when simultaneous bidirectional communications are required, as is the case with computer modems.
Perhaps the most widely used technique of echo cancellation utilizes a feedback tap from the hybrid interface circuitry to cancel the echo signal, as shown in FIG. 3. The hybrid is adjusted so that the feedback signal at node N0 closely matches the echo signal at node N1 so the feedback signal can be utilized to fully cancel the echo signal. This type of adjustment is commonly referred to as xe2x80x9cbalancingxe2x80x9d the hybrid. The optimum echo cancellation is obtained when the ratio of RFB/Zvar exactly matches the ratio of ROUT/Zline because in this xe2x80x9cbalancedxe2x80x9d state, the voltage at the output node N1 is equal to the voltage at the feedback tap N0, thus facilitating echo signal cancellation without impeding the propagation of west-east signals received at the node N0.
The value of Zvar which results in the best balance of the hybrid is of course equal to (or proportional to) Zline. However, because Zline is a complex parameter derived from transmission line characteristics involving reactive and resistive impedances distributed along the transmission line, it is usually modeled as a simpler circuit using lumped circuit elements. In this way, a circuit realization for Zvar that corresponds generally to the estimated Zline is more easily implemented. Various transmission line models have been used within four-wire to two-wire hybrids to reduce echos. Prior art devices which use this type of echo cancellation balance the hybrid by adjusting variable resistors, capacitors and/or inductors in the presence of a tone, typically 2 kHz, which is applied to the hybrid to mimic a transmitted signal. The amount of signal cancellation is then measured using one or more voltage comparators, correlators, or other comparison circuits to achieve the best echo signal cancellation of which the device is capable.
The inventors named herein discovered that the use of a narrow-band signal to balance the hybrid does not result in adequate echo cancellation when the communications circuit is in operation because the impedance of the line varies significantly as a function of frequency. The use of a single tone to balance the hybrid only ensures that frequencies only in that part of the spectrum will be canceled effectively. Previously, this single-tone technique has resulted in acceptable echo cancellation performance for voice communications. Recently, with the wide use of data communications devices, a need for a more accurate balancing has arisen.
Additional prior art line testing devices (known as a xe2x80x9ctest setxe2x80x9d, typically used by the phone company technician) use white noise for the calibration signal, but this is also not a good method, as it requires an oscillator or noise generator to make the signal, thus requiring additional components. Additionally, the noise power spectrum has a flat amplitude, and is not weighted to reflect the transmissibility of the voice band.
The present invention uses multitone broad-band measuring of the line impedance properties to provide for a more complete and comprehensive echo reduction over the entire voice band spectrum. Furthermore, the present device accomplishes this task without introducing additional components necessary just for the line balancing. Further, the amplitude of the signals is weighted to reflect the full voice band.
Two prior art patents that disclose line conditioner devices include U.S. Pat. Nos. 4,961,218 and 5,504,811.
The present invention provides a two-wire line conditioning apparatus having an improved calibration and cancellation circuit and provides an improved calibration method using a multitone calibration signal. The apparatus automatically adjusts an electronic hybrid as part of a calibration procedure so as to eliminate undesirable echo effects when calls are placed along the line. The device provides conditioning for 2-wire switched special service circuits including PBX trunks, WATS, analog data circuits and plain-old telephone service (POTS), as shown in FIG. 1. The electronic hybrid balancing process and echo cancellation circuitry may also be utilized in a circuit for converting four-wire communications systems to two-wire communications systems.
The line conditioning device of the present invention utilizes a multitone signal to simulate the voice frequency spectrum between approximately 500 Hz and 3500 Hz (see FIG. 4, a log plot of C-message load, which provides an indication of which frequencies are more critical to an average listener) to probe the line side with a plurality of tones (e.g., 500, 2000 and 3500 Hz; or 500, 1000, 1500, 2000, 2500, 3000, 3500 Hz). The amplitude of the tones are weighted to reflect the characteristics of modems and voice in the voice band, as shown in FIG. 6. The value of Zvar considered to be optimal is that value which minimizes the power of the echo of the weighted multitone signal.
The multitone calibration signal is preferably generated by band pass filtering a rectangular pulse train. This method of generating the calibration signal is simple and economical because it requires relatively few components. By regulating the bandpass filter characteristics and/or the period and duty cycle of the pulse train, the set of frequencies used to analyze echo may be optimized. The weighted multitone signal is injected into the east-west signal path into the electronic hybrid to mimic a signal originating from the customer. The multitone signal is transmitted along the line, but is also simultaneously detected by the initial west-east amplifier within the electronic hybrid. This feedback signal is combined with the cancellation signal from the other branch of the electronic hybrid containing Zvar. The amount of cancellation obtained is measured by a power level detector. In response to the detected level, the variable impedance Zvar of the electronic hybrid is adjusted so as to obtain the minimum power level measurement of the echoed multitone signal. One advantage of using multiple weighted tones over a single frequency or a broadband noise signal is that echo cancellation is significantly improved over the specific range of frequencies of interest. Furthermore, the improved line balancing method and apparatus does not require additional componentry.